Organic thin film transistors have lots of excellent features such as they can be produced by employing low-temperature film formation process, it is easy to form a film on a flexible and light-weight substrate made of resins or the like, and they are suited for inexpensive solution application processes, in compared with thin film transistors using the conventional inorganic silicon thin film. Their research and development are actively preceded as the core technology for next generation of flexible electronics.
FIG. 2 is an outline cross-sectional view of a top-contact type organic thin film transistor which has a representative structure of an organic thin film transistor (hereinafter often referred to simply as an “organic TFT”).
The top-contact type organic TFT 2 has, on a substrate 10, a gate electrode 20, an insulator layer 30 and an organic semiconductor layer 40 in this sequence. A drain electrode 50 and a source electrode 60 are disposed on the organic semiconductor layer 40 with a predetermined distance (channel region 70).
Like the organic TFT 2, such a structure in which a source electrode 60 and a drain electrode 50 are arranged opposite a substrate 10 via an organic semiconductor layer 40 is particularly called as a top-contact type.
In the top-contact type organic TFT 2, the organic semiconductor layer 40 forms the channel region 70, and the flow of an electric current to the source electrode 60 and the drain electrode 50 can be controlled by the voltage applied to the gate electrode 20.
The top-contact type organic TFT 2 can be fabricated as follows: on the substrate 10 on which the gate electrode 20 and the insulator layer 30 are formed, the organic semiconductor layer 40 is formed by vacuum deposition or a solution process such as spin coating, dip coating or casting, and the source electrode 60 and the drain electrode 50 are formed by, for example, vacuum deposition using a deposition mask.
FIG. 3 is an outline cross-sectional view of a bottom-contact type organic TFT.
The bottom-contact type organic TFT 3 has, on a substrate 10, a gate electrode 20 and an insulator layer 30 in this sequence. A drain electrode 50 and a source electrode 60 are disposed on the insulator layer 30 with a predetermined distance (channel region 70), and an organic semiconductor layer 40 are further stacked on the drain electrode 50 and the source electrode 60 to cover these electrodes.
Like the organic TFT 3, such a structure in which the source electrode 60 and the drain electrode 50 are formed on the substrate, and the organic semiconductor layer 40 are stacked on the source electrode 60 and the drain electrode 50 is particularly called as a bottom-contact type.
In the bottom-contact type organic TFT 3, the gate electrode 20, the source electrode 60 and the drain electrode 50 form a circuit pattern on the substrate 10, and the organic semiconductor layer 40 is formed on the circuit pattern.
For forming an electrode, known photolithography and the like can be employed. Therefore, a circuit pattern having high-resolution and a large area can be easily formed. Thus, different from the top-contact type organic TFT, in the bottom-contact type organic TFT, the organic semiconductor layer is formed on the substrate on which the circuit pattern has been previously formed. As a result, the bottom-contact type organic TFT has the advantage that the organic semiconductor material constituting the organic semiconductor layer does not deteriorate due to the physical and chemical stresses associated with the formation of electrodes.
The bottom-contact type organic TFT has the above-mentioned advantages. However, there is a problem that the properties of the bottom-contact type organic TFT are significantly inferior to a top-contact type organic TFT fabricated using the same organic semiconductor material. It is considered that in the bottom-contact type organic TFT, a large contact resistance lies between the organic semiconductor layer and the electrodes.
To solve the problem, an approach to use a multilayer structure composed of an oxide layer/metal layer for the source-drain electrodes. Namely, this approach is aimed to lower the charge-injection barrier between the electrode layer and the organic semiconductor layer using materials having a good charge-injection property for the oxide layer.
Patent Document 1 discloses a TFT wherein source-drain electrodes contact with a p-type organic semiconductor via a charge-injecting layer formed of an inorganic substance, an oxide of molybdenum or vanadium is used for the charge-injecting layer, and the inorganic charge-injecting layer having a medium energy level is disposed between the channel and the source-drain electrodes.
The effects of this TFT include decrease of the driving voltage, stabilization of the properties and increase of the reliability.
Non-Patent Document 1 discloses a TFT which has a MoOx/Au electrode using MoOx in place of Cr or Ti generally used for a base layer for the Au electrode. Further, it describes that the thickness of the MoOx is preferably 2 nm.
This TFT can reduce the contact resistance between the source-drain electrodes and the organic semiconductor (pentacene) to decrease the voltage.
Different from the above-mentioned approach, an approach wherein a metal electrode is surface-modified with an organic compound having a thiol group at the terminal to form an organic thin film layer, thereby controlling the wettability of the surface of the metal electrode and the work function has been also made.
Patent Document 2 discloses a bottom-contact type TFT wherein a taper (incline) is provided with the edge portion of sourced-drain electrodes, the width of the taper is made to be smaller than the average particle diameter of semiconductor crystals, and an organic compound layer (1 Å to 10 Å) formed of a compound having a thiol group is disposed between the source-drain electrodes and the semiconductor layer.
In this TFT, the contact resistance of the interface of the source-drain electrodes/semiconductor is reduced to increase the performance.
Patent Document 3 discloses a TFT wherein thiocresol is disposed between the source-drain electrodes and the organic semiconductor film.
Patent Document 4 discloses a TFT wherein an electrode-surface treating agent having a functional group (for example, a thiol group) which forms a chemical bond with a metal is used.
This TFT has good TFT property, and solution processes which give high production efficiency can be employed.
Non-patent Document 2 discloses a TFT wherein an Au electrode is treated with a SAMs (self-assembled molecular film) such as decanethiol, CH3—(CH2)9—SH (DT), perfluorodecanethiol CF3—(CF2)7—(CH2)2—SH (PFDT) or perfluorohexanethiol CF3—(CF2)3—(CH2)2—SH (PFHT), to vary the work function so that charge injection from the electrode is improved.
In this TFT, for example, for the DT/Au electrode, the work function decreases by 0.45 eV, and for the PFDT/Au electrode, the work function increases by 0.9 eV. Accompanying thereto, the contact resistance increased in the case of the DT/Au electrode and decreased in the case of the PFDT/Au electrode, in comparison with the Au alone.
Non-patent Document 3 discloses a TFT wherein the hole-injection barrier is lowered by treating a Cr/Au electrode with 1-hexadecanethiol (CH3—(CH2)15—SH), and which has the following relationship:Ip:Au(5.1 ev)>Pentacene HOMO(5.0)>Au/C16H33SH(4.9).
Non-patent Document 4 discloses a TFT wherein a gold electrode is treated with pentafluorothiophenol (PFTP) to improve the contact between the electrode-semiconductor.
In spite of the above-mentioned approaches, the problems in the bottom-contact type organic TFT of high threshold voltage, low mobility and the like cannot be completely dissolved.
An object of the invention is to provide a bottom-contact type organic TFT having a low threshold voltage, a high field-effect mobility and a high on/off current ratio.